1. Field of the Invention
The present invention relates to a method and apparatus for measuring the shape of a sheet and, more particularly, to a method and apparatus for performing online measurement of the shape of a sheet of a planographic printing plate provided with interleaving paper by using an optical measuring device.
2. Description of the Related Art
In recent plate making methods (including electrophotographic plate making methods), planographic printing plates such as photosensitive printing plates and thermosensitive printing plates are widely used to facilitate automatization of the plate making process. A planographic printing plate is generally manufactured as described below. One of surface treatments including sand dressing, anodization, a silicate treatment and other chemical treatments or a suitable combination of such treatments are performed on a supporting member such as an aluminum plate in the form of a sheet or a coil. Then, film coating treatment for forming a photosensitive layer or a thermosensitive layer (hereinafter referred to as a printing surface) on the supporting member, and drying treatment of the coating material are performed. The sheet thus formed is fed to a sheet working process and is cut by a predetermined slit width with a slitter and is thereafter cut by a predetermined cut length with a flying cutter, thus manufacturing a planographic printing plate in the form of a rectangular sheet.
To guarantee the product size of the cut planographic printing plate, in this sheet working process, sampling of the planographic printing plates is performed after cutting the plates in the form of sheets for, and the shape of the planographic printing plates, e.g., the product size (the slit width, the cut length, the perpendicularity) is measured by offline inspection, thereby enabling the provision of a guarantee of the size of the planographic printing plate. A certain number of operator steps are required for the offline inspection. Also, if a size abnormality is detected by the offline inspection, a one hundred percent inspection on the planographic printing plates having been cut before the completion of the offline inspection is required. If all the products inspected by the one hundred percent inspection have size abnormalities, a considerable loss, i.e., a product loss of a larger number of planographic printing plates, results.
In recent years, with the implementation of schemes to increase the sheet working processing speed, schemes to seal digital products in darkrooms, schemes to save manpower, etc., there has been a demand for introducing online measurement apparatuses capable of automatization of various visual process inspections and online real-time measurement.
An apparatus for online measurement of the shape of a sheet, e.g., one disclosed in Japanese Patent Application Publication No. 5-52526 or one disclosed in Japanese Patent Application Publication No. 6-147836 has at least four sets of CCD cameras and stroboscopic devices (Japanese Patent Application Publication No. 5-52526) or at least four sets of transmission-type photosensors and stroboscopic devices (Japanese Patent Application Publication No. 6-147836) positioned in correspondence with four sides or four corners of a conveyed sheet so that the four sides or four corners of the sheet can be imaged at a time, and instantaneously images the sheet by synchronizing the CCD or the sensor and the stroboscopic device with arrival of the sheet at each sensor position.
The online shape measurement apparatuses disclosed in Japanese Patent Application Publication Nos. 5-52526 and 6-147836, however, have a drawback in that it is difficult to accurately synchronize the plurality of CCD cameras or transmission-type photosensors, and a measurement error can occur easily under the existing circumstances where the sheet conveyance speed (line speed) in the sheet working process is increased.
In the case of a planographic printing plate formed from a sheet having a photosensitive layer, there is a possibility of occurrence of fog in the photosensitive layer due to the strobe lighting. Further, since at least four sets of CCD cameras (or transmission-type photosensors) and stroboscopic devices are provided, the overall size of the apparatus is increased and the measurement apparatus cannot be used if the installation space is not sufficiently large.
Furthermore, if the sheet changes in attitude by meandering of the sheet conveyor or floats due to curl at a sheet corner or flutter of the sheet during online measurement on the sheet shape, or if the accuracy with which image pickup devices for imaging the sheet shape are mounted is low, an accurate sheet shape measurement cannot be made. To make an accurate online sheet shape measurement, therefore, it is necessary to solve the problem that the sheet changes in attitude and floats and the problem relating to the accuracy with which image pickup devices are mounted as well as the problems relating to Japanese Patent Application Publication Nos. 5-52526 and No. 6-147836.